Hi everyone, So I have always been interested in space, and when my college offered an intro to astrophysics course with no prerequisites, I jumped at the opportunity. I had already had a lower level astronomy/physics course, and physics and astronomy in high school, so I thought I'd be fine, but am now finding that everything is a much bigger struggle than presupposed! I learn quickly and just need to find access to some basic concepts, as my grasping at remenants from highschool physics is not really getting me anywhere. Here is a problem that I understand loosely how to set up, but I don't quite know which formula to use or which values to put where: An astronaut in a starship travel to a Centauri, a distance of approximately 4 ly as measured from Earth, at a speed of u/c=0.8. d.) A radio signal is sent from earth to the starship every 6 months, as measured by a clock on Earth. What is the time interval between reception of one of these signals and reception of the next signal aboard the starship? In previous parts of the problem I solved that the trip to "a Centauri" takes 5 years, as measure by a clock on Earth, 3 years, measured by the pilot, and the distance between the two measured by the pilot was 2.4 ly. Now, I realize that I need to figure out how long it will take a radiowave to the starship at distance A, then how far the starship travels in 6 months, then how long it will take a radiowave to travel distance B. Radiowaves travel at the speed of light, and the starship is traveling at a speed of u/c=0.8. I believe I need to use some manipulation of the formula: y=1/(sqrt(1-(u^2)/(c^2))) (excuse the "y" instead of the appropriate symbol, working with a keyboard, you know how it goes) But I am just not sure how to manipulate, or what exactly to put where...and i'm sure it's an extremely intuitive problem, I'm just messing up/not seeing little things. Any help would be appreciated! Also if anyone has any helpful facts about online astrophysics tutoring, or anything, please let me know!